Dark trace cathode-ray tube and method of manufacture



May 27, 1958 HQLBQRN' ETAL 2,83$,754

DARK TRACE CATHODE-RAY TUBE AND METHOD OF MANUFACTURE Filed Oct. 6, 1954 70 Palm/ 4 IN V EN TORS. FEED'E/CK H01. EOEN/fleceaserfl United States DARK TRACE CATHGDE-RAY TUBE AND METHOD OF MANUFACTURE Frederick Holborn, deceased, late of Cedar Grove, N. 3., by Kate Holborn, executrix, Cedar Grove, and Gregory Hodowanec, East Orange, N. 3., assignors to National Union Electric Corporation, Hathoro, 1%., a corporation of Delaware Application October 6, 1954, erial No. 469,624

11 Claims. ((31. 3113-92) This invention relates to electron tubes of the electronoptical transducer kind, and more especially it relates to tubes of the so-called dark trace variety, especially those employing an alkali-halide crystal screen as the transducer element. Such transducer elements are referred to herein as scotophors.

A principal object of the invention is to provide an improved cathode-ray tube of the scotophor screen kind, having a high degree of optical contrast and capable of rapid record erasure.

The screens of such dark trace tubes become magneta in color when exposed to cathode rays. This is due to the creation of an absorption band for visible light which is attributed to the trapping of electrons by anion vacancy sites within the crystal. These trapping sites are commonly termed F-centers and the absorption band is termed the F-band.

The chlorine ions in the potassium chloride, which represents the preferred form of scotophor material, lose electrons under electron bombardment and become essentially neutral chlorine atoms. These chlorine atoms are generally termed holes in the crystal. The ejected electrons are free to move within the crystal, and, at room temperature, there is a high probability that they will be trapped by the electrostatic fields of anion vacancies as F-centers.

At room temperature the F-center has two prominent metastable states. There is the lowest energy or ground state and a first excited state. The excitation of the F- center electron from the ground state to the first excited state by quanta in the visible spectrum gives rise to the absorption band called the F-band. While the F-center is in the excited state, it requires but a small additional amount of energy to be ejected from the trap and thus be free to move Within the crystal. This energy can be supplied by the agitation of the ions surrounding the F-centers. When the F-centers are made sufficiently unstable by surrounding ion agitation, then the electrons ejected frcm F-centers are forced to return to holes in the crystal. The crystal now reverts back to its original uncolored state, i. e. erasure takes place.

The primary object of this invention is to provide a dark trace cathode-ray tube using a simple internal crasure mechanismwhich is not destructive of the alkalihalide screen.

Another object of this invention is to provide a dark trace tube which has an erasure mechanism which is uniform and constant in action.

A still further object is to provide a dark trace cathode-ray tube which has an essentially white screen of long life in conjunction with an erasure mechanism which is reliable, rugged, and which operates with a simple power source.

The above and other objects may be accomplished by practicing this invention which makes use of the infrared absorption characteristics of potassium chloride and ice carbon black in a manner to be hereafter more fully disclosed.

The salient point in this invention is a composite radiation-converting layer applied directly to the alkali-halide of the dark trace tube screen. This composite radiationconverter layer, while absorbing but an inappreciable amount of cathode-rays, appreciably whitens the appearance of the tube screen.

A feature of the invention relates to a scotophor screen which has, as an integral part thereof, a radiation wavelength converter for converting received radiations in the short infra-red range, for example approximately 7000 A.-14,000 A., into long infra-red region, for example 20 to 10911., whereby erasure heat energy can be efficiently and rapidly applied directly to the scotophor material Without destroying its transducer qualities and without deleteriously affecting its life.

Another feature relates to a scotophor screen having a light-transparent backing for a scotophor, and a multilayer coating on the scotophor acting as a wave-length converter for incident infra-red radiation, one of the layers of said coating consisting of a light-weight metal such as aluminum, beryllium, magnesium, and a layer of amorphous carbon in the form of soot or carbon black.

Another feature relates to a scotophor screen having a light-transparent backing for a scotophor, and a multilayer coating on the scotophor acting as a Wavelength converter for incident infra-red radiation, one layer of said coating consisting of a light-Weight metal transparent to a cathode-ray beam and in crystalline form, and another layer in the form of soot or carbon black also transparent to the cathode-ray beam.

A further feature relates to a dark trace cathode-ray tube having a scotophor screen which carries as an integral part thereof an infra-red wavelength converter including a light-weight metal and carbon black, in conjunction with a line wire filament metal inside the tube adjacent the screen for irradiating the screen with relatively short Wavelength infra-red, e. g. 7000 A.-14,000 A., and with the inside wall of the tube provided with a mirror coating for more efliciently irradiating the said screen with said infra-red.

A still further feature relates to the novel organization, arrangement and relative location and composition of elements which cooperate to form an improved scotophor screen capable of rapid erasure.

Other features and advantages not particularly enumerated will be apparent after a consideration of the following detailed descriptions and the appended claims.

In the drawing,

Fig. 1 is a longitudinal sectional view of a dark trace cathode-ray tube, embodying the invention.

Fig. 2 is a magnified cross-sectional View of part of the scotophor screen of Fig. 1, taken along the line 2-2 thereof.

Fig. 3 is a sectional view of Fig. 1, taken along the line 33 thereof.

Fig. 4 is a schematic diagram of a typical apparatus for controlling the vacuum deposit of the various screen coatings.

in the drawing, which shows by Way of example one preferred embodiment, the numeral 19 represents any well-known form of evacuated bulb such as conventionally used in cathode-ray tubes. The said bulb has the usual neck portion 11 in which is mounted any well-known form of electron gun 12 for developing a focused electron beam 13. The tube is also provided with any Well-known coordinate beam deflecting means represented schematically by the horizontal and vertical deflection plates 19, 20. The front or face end 21 of the tube is of glass and suitably mounted adjacent that end under controlled conditions.

This screen may be supported in an armular or rectangular metal frame 23 which can be fastened in any suitable way against the end wall 21-.

As shown in magnified sectional view in Fig.2, the screen 22com'prisesanalkali-halide crystal material 24, preferably potassium chloride, which is sprayed, settled, orvaporize'd in vacuum. on athin mica or glass lighttransparent supportbacking 25, which is held in. frame 23in a ry suitable manner. For example, the backing 25 may have a thickness of about ;00l inch. A thin amorphous or quasi-amorphous coating ofa light-Weight metal 26,- such as aluminum, beryllium, or magnesium, is then depositedonthe; scotophor 24. The depositing of this light-weight metal should be done in a vacuum 7 For. example, as schematically shown in Fig. 4, the deposition of the scotophor materiah'such as potassiumchloride, on the backing .25 can beetlectcd -by placingthe frame 23 and th'ebacking 25 in a supportingfiig 28 inside the bell jar and the potassium chloride canv be vaporized. from a suitably heated'cup'33, to form the scotophordepositp .Cupf33 can --be heated by current applied to lead-ins 33a, 3%. Similarly,- the light-weight metal coating 26 can be deposited over the scotophor in the evacuated Jbell jar. For example, there may be mounted within the bell jar a metal filament 29 carrying a series of pellets 30 of the desired light-weightmetal. The filament 29 can be connected by mean s of suitable lead-in members 31, 32,'to a sourceof heating current so as to flash or vaporize the pellets St on to the scotophor. p

In accordance with the invention the vaporization of this light-Weight metal is done in successive steps so that the layer 26 is initially deposited as a stratum 25a of the light-weight metal in amorphous form, and thenthe next stratum 26b of this vaporized light-weight metal is de-' posited as a mirror deposit which is highly reflective to visible light rays and is .a good metallic conductor. In other words, the stratum 26b is crystalline in character, whereas the stratum 26a is amorphous. Stratum 26a is deposited by vaporizing'the light-weight metal against the rough'surface of thescotophor 24 in a relatively poor vacuum of say approximately X 10- mm. of Hg in pressure whilefor stratum 26b, the evaporation takes place in a vacuum of approximately 10 mm. of Hg in pressure or better. The evaporation of the aluminum is discontinued when the crystalline deposit is just barely trans lucent in an intense source of white light. Finally, a thin but black deposit of pure carbon black or soot 33 is deposited on the stratum 26b. This soot can be applied by exposing the stratum 26b to aniatmosphere resulting fro'mth'e incomplete combustion of a mixture of methane and benzene.

We have found it is important that the amorphous. carbon coating referred to hereinabove should. be in the form of substantially pure carbon black or soot. soot can be produced by the incomplete combustion of is a high velocity beam of the order of 8' to 14 kilovolts,v 0

' long wavelength infra-red, for

the manufacture of cathode-ray tubes, because carbon black is relatively inactive at the temperatures to which a it may be exposed during such processing.

It should be understood that the thickness of the various coatings 2.4, 26 and 3-3 should be controlled and limited so that their combined thickness is transparent to the electron beam 13. For example, if the beam 13 the thickness of the scotophor 24 may bell to 10 microns; the thickness of the double strata layer 26 can be 0.3 to 0.5 micron; and the thickness of the carbon black 33 can be 0.5 to 2.0 microns. I i

We have found that when such a screen is subjected to the oscillating cathode-ray beam 13,.there is produced in the scotophor 2d a record of the signal modulations which occur in the beam-13, and it is possible to erase this record comparatively rapidly by subjecting the screen 22 to im"ra-red radiation inthe relatively short wavelength range, for example 7000 A.l4,000 A. However, as a result of the composite character of. the coatings 26 and 33, they act as a frequency converter for this relatively short wavelength infra-red and convert it into relatively In order to effect this rapid erasure, therefore, it

necessary to provide within the tube 10 an 'eificient and stable source of the short'wavelength infra-red and this source should be such that it does not deleteriously ample at a distance of 2 to. 4 inches therefrom,

' supply line. 'For example, the filament 3 4 may'consist of a tungsten wire of approximately 0.008inch diameter which, 'whenconnected to a 115, volt supply line, rises very rapidly to a temperature at which it becomes an eilicicnt source of infra-red radiation, for example in the range from 7000' A.l4,000 A., corresponding to a filament temperature of approximately 2 100 K. In otherv words, the filament converts the filament heating current mainly into radiated energy. in" the short wavelength infra-red range. Preferably," the filament 34. is zig-zag inshape so as to "substantially uniformly irradiate the screen 22 over its entire surface :and because of the fineness of the filamentfit casts negligible electronshadow on the screen as the beam 13 is scanningthescreen. If

desired, this liability to shadow may. be ,evenifurther reduced by biasing the screen negatively with respect to the filament 34 by a; suitable adjustable source schematically represented by the battery 37. It will be understood'that This illuminating gas (namely 'CH and benzene vapor (C l-I The gas can be. bubbled through a wash bottle containing benzene. The soot canbe deposited at the screen either by holding the screen in the gas flame or supporting the screen above the flame such asin a bell jar. The soot'thus deposited is .a heat absorber and transfers its heat energyby conduction to the light-weight metal deposit which reradiates the energy as long wave infra-red radiation. The long wavelength infra-red is readily absorbed by the scotophor material which is,

however, diathermic for infra-red below 20 microns. p

This soot layer 33 has very high absorption for radia radiation of wavelength tion in the visible or inner infra-red spectrum. It has the additional advantage that it requires no special precau-' tions when the screen is i'finally assembled in the tube 10 and 'that tube is subjected to the usual processing and exhaust schedules such as are conventionally used in bya suitable switch (not shown) thecurrent is applied to filament 34 only when thebeam 13 is blanked off during theerasure period. The beam canbe blanked off by applying a cut-oil bias to the control grid of the electron gun in known manner. p

In order further 'to increase the efficient infra-red radiation to the screen, the entire bowl or bulb portion-ofthe tube 10 can be coated on its innersurfacewith a layer 38in the form of a mirror-like coating of aluminum.

This coating 38 also serves to keep thefbowl or'bulb" portion of the tube relatively cool during the erasure cycle. with the metal frame 23. The neck portion of the bulb 10 can be provided with the usual non-reflecting or -coll loidal graphite coating 39. V a

' The operation of the erasure cycle can :be described as follows. The visible and short wavelength infra-red radiation from the tungsten filament 34 is directly ab.-

sorbed bythe soot layer 33 on the screen, The soot thereupon heats the metallic layer 26 Whichin turn acts as an infra-red converter and radiates-long wavelength infra-red with relatively high'efiiciency from the quasi example in the range of Preferably the coating 3,8;extends into cohtact' amorphous stratum 26a which is in direct contact with the scotophor 24-. The scotophor 24, therefore, directly absorbs this long wavelength infra-red radiation in the process of volume polarization of its crystals. Suflicient energy is thus absorbed by the scotophor to efiect erasure of the F-centers in the crystal. There is additional absorption of energy from the radiation source and by heat conduction during the interval that the electrons are mobile in the conduction band of the crystal. During this interval, the scotophor crystal has metallic characteristics, while normally it is a good insulator. One of the advantages of constituting the metallic layer 26 of a double strata coating is that the amorphous stratum 26a is a relatively poor heat conductor While the crystalline stratum 26b is a good heat conductor and thus more efiiciently converts the heat adsorbed by the carbon black layer 33 into the desired infra-red radiation for erasing the record on the scotophor 24.

Various changes and modifications can be made in the disclosed embodiment without departing from the spirit and scope of the invention. By the expression carbon black, as used herein, is meant powdered carbon as distinguished from pelletized or crystalline carbon. Such carbon blacks are referred to in the trade by various names, such as channel blacks, furnace blacks, lamp blacks, acetylene blacks, soot, flame blacks, and the like, as distinguished from crystalline carbon or graphite.

What is claimed is:

1. Cathode-ray tube apparatus of the dark trace scotophor kind comprising in combination, an enclosing envelope, means to develop a beam of electrons, a screen upon which said beam impinges to make a record, said screen including a scotophor material which develops opacity centers when said beam impinges thereon, and means to erase said centers, the last mentioned means comprising a source of infra-red radiation, a layer of carbon black on said screen facing said beam developing means, and a layer of metal from the group consisting of aluminum, beryllium and magnesium, sandwiched between said carbon black and said scotophor, said layers of carbon black and metal having a total thickness which is transparent to said beam.

2. Cathode-ray tube apparatus according to claim 1, in which said metal layer is composed at least in part of a stratum in amorphous form.

3. Cathode-ray tube apparatus according to claim 1, in which said metal layer is composed of two strata, one of which is amorphous and the other crystalline.

4. Cathode-ray tube apparatus of the dark trace scotophor kind comprising in combination, an enclosing envelope, an electron gun to develop a beam of electrons, a screen upon which said beam impinges to make a record, said screen comprising a scotophor which develops opacity centers when said beam impinges thereon, a source of infia-red radiation within said envelope between the gun and screen, and an infra-red wavelength converter forming an integral unit with said screen, said converter comprising a layer of carbon black facing the gun and a layer of amorphous aluminum between the carbon black and the scotophor.

5. Cathode-ray tube apparatus according to claim 4 in which said infra-red source is a fine wire refractory filament having lead-ins for supplying it with heating current to raise it to a temperature at which it efficiently radiates infra-red in the region between 7,000 A- l-i,000 A.

6. A screen for dark trace cathode-ray tubes, comprising a transparent thin backing support, a coating of scotophor material on said support, and means to convert incident short Wavelength infra-red radiation into long Wavelength infra-red radiation for record erasure, said means comprising a layer of amorphous carbon, and a layer of a metal from the group consisting of aluminum, beryllium and magnesium the last-mentioned layer being located between the scotophor and the first-mentioned layer.

7. A screen for dark trace cathode-ray tubes, comprising a transparent thin backing support, a coating of scotophor material on said support, a coating of metal from the group consisting of aluminum, beryllium and magnesium, on said scotophor, and a layer of soot on said coating for the purpose described.

8. A screen for dark trace cathode-ray tubes, comprising a transparent thin backing support, a coating of scotophor material on said support, a stratum of amorphous aluminum in direct contact with said scotophor, a stratum of crystalline aluminum in direct contact with the amorphous stratum, and a stratum of carbon black on said crystalline aluminum stratum, said strata having a total thickness which is transparent to an incident cathode-ray beam while converting incident short wavelength infrared radiation into long wavelength infra-red radiation.

9. The method of making a scotophor screen for a dark trace cathode-ray tube and the like, which comprises depositing a scotophor on a light transparent backing, applying to said scotophor a coat of an element from the group consisting of aluminum, magnesium and beryllium, then applying a coating of carbon black to said element and limiting said element and carbon black to a thickness which is transparent to an incident cathode-ray beam while of sufficient thickness to act in conjunction with said element as a wavelength converter to convert incident short wavelength infra-red radiation to longer wavelength infra-red radiation.

10. The method according to claim 9 in which said element is applied in a vacuum to form it with two successive strata, one of which is amorphous and the other crystalline.

References Cited in the file of this patent UNITED STATES PATENTS 2,533,381 Levy Dec. 21, 1950 2,545,200 Fonda Mar. 13, 1951 2,615,821 Levy Oct. 28, 1952 2,616,057 Coltman Oct. 28, 1952 2,661,437 Beckers Dec. 1, 1953 2,665,220 De Gier Jan. 5, 1954 

6. A SCREEN FOR DARK TRACE CATHODE-RAY TUBES, COMPRISING A TRANSPARENT THIN BACKING SUPPORT, A COATING OF SCOTOPHOR MATERIAL ON SAID SUPPORT, AND MEANS TO CONVERT INCIDENT SHORT WAVELENGTH INFRA-RED RADIATION INTO LONG WAVELENGTH INFRA-RED RADIATION FOR RECORD ERASURE, SAID MEANS COMPRISING A LAYER OF AMORPHOUS CARBON, AND A LAYER OF A METAL FROM THE GROUP CONSISTING OF ALUMINUM, BERYLLIUM AND MAGNESIUM THE LAST-MENTIONED LAYER BEING LOCATED BETWEEN THE SCOTOPHOR AND THE FIRST-MENTIONED LAYER. 